For many purposes it is of value to use more than one imaging method to construct images of a subject, and to compare the different images. Until recently an image would be taken one system—for example using X-rays, and at another time, and perhaps at another location, another image would be acquired, for example by a radioisotope imaging system using gamma rays.
A problem with using more than one image is the issue of alignment. If two images are not aligned properly, then the comparison of findings from one image with those of another image will be faulty. Thus true correlations between some details will be missed and false correlations and/or other artifacts are likely to be encountered.
Recently multi-modality imaging systems have become available, in which more than one imaging system use a common patient-support system. In many such machines the imaging equipment of the different systems is held by one or more gantries. The subject is supported and moved from one imaging area to another imaging area by a support system, often comprising a supporting element upon which the subject lies, for example a stretcher, and a base, or table, which holds, supports and controls the motion of the supporting element.
In medical imaging, the part of the patient's body (in some situations: the whole patient) to be scanned (herebelow: the subject) is generally substantially the same for all the imaging systems. The stretcher, on which the patient lies, is differentially extended from the table into the gantry volume, to position the subject in the different imaging planes. The stretcher, which is generally cantilever supported, sags differently at each imaging plane, because it is differentially extended. As a result of the differential sag, the images from the various imaging systems are not properly aligned. While this is not a problem for simple image viewing, it may be a problem for systems in which the images interact, as for example where a CT (X-ray computerized tomography) image is used for attenuation correction for a NM (Nuclear Medicine) gamma ray image. Such corrections are well known in the art.